With respect to a conventional method for triggering IF measurements in a mobile communication system, FIG. 1 shows a general overview of a telecommunication system TELE which comprises at least two different mobile communication systems T1, T2. A subscriber station, e.g. a mobile station MS, which is operable in the first mobile communication system T1, may also be operable in the second mobile communication system T2. Within each mobile communication system T1, T2 the mobile station MS can move around different cells S1, S2, S3, S1′, S3′ and C1–C6. Due to different handover criteria the mobile station MS may perform an inter-frequency handover within the same system or an inter-system handover to/from the other system. It should be noted that the present invention is equally well applicable for triggering an inter-frequency handover within the same system and/or an inter-system handover and FIG. 1 only shows two mobile communication systems T1, T2 as an example where both such handover procedures may take place.
FIG. 1 shows as an example for the first mobile communication system T1 a WCDMA (Wideband Code Division Multiple Access) or CDMA (Code Division Multiple Access) communication system comprising a network control means RNC (Radio Network Controller), at least one base transceiver station RBS, RBS′ (in WCDMA called radio base station), at least one subscriber station MS (Mobile Station) as well as a number of (possibly) overlapping cells S1, S2, S3, S1′, S3′.
An example for the second mobile communication system T2 is a communication system according to the GSM (Global System for Mobile Communications), PDC (Personal Digital Cellular) and D-AMPS (Digital-Advanced Mobile Personal Service) standards.
In FIG. 1 an example of a GSM system is shown for the second mobile communication system T2. However, it should be noted that the invention can in principle be applied to any type of digital mobile telephone system and is as such not restricted to the aforementioned systems. The GSM system shown in FIG. 1 comprises the conventional units of a base station controller BSC, at least one mobile switching center MSC as well as a gateway mobile switching center GMSC. The mobile stations MS are served by a plurality of base transceiver stations BTS within the cells C1–C6 in which the mobile station MS can move around.
The network control means RNC of the WCDMA system in FIG. 1 is connected via a UMSC unit to the gateway mobile switching center GMSC of the GSM system.
Depending on the geographical layout of the first and second mobile communication systems T1, T2 the cells S1, S2, S3, S1′, S3′ of the first mobile communication system T1 may also completely or partially overlap with the cells C1–C6 of the second mobile communication system T2. Of course, if the mobile station MS is to carry out an inter-system handover—then the mobile station MS will be able to operate according to the specifications of the first and the second mobile communication system.
One reason for performing inter-frequency or inter-system handovers in the telecommunication system TELE in FIG. 1 may be due to coverage reasons. This is due to the fact that neither the first communication system nor any other system has a complete coverage in all geographical areas, e.g. hot spots in UMTS. Furthermore, some cells within the mobile communication system may operate on frequencies which are not applicable in adjacent cells. Therefore, by letting the mobile station MS or the network control means RNC perform either an inter-frequency handover of an inter-system handover, the mobile station MS can be used in a larger area without interruptions in the communication.
Another reason for the handover may be capacity reasons. Either the mobile communication system or other mobile communication systems may become heavily loaded at times, so that an inter-system handover may be required. Analogously, the mobile station MS may have established a connection on a particular frequency and it may be necessary that another frequency is to be used. This other frequency may be present within the same cell or in another cell and both are generally termed inter-frequency handover. As indicated in FIG. 1, the inter-frequency measurements (necessary for an inter-frequency handover/or an inter-system handover) is always carried-out by an inter-frequency measurement means IFMM situated in a mobile station MS.
The network control means RNC comprises a paging flag sending means PFSM for sending a paging flag to the mobile station MS when a signaling communication link has already been established between the subscriber station MS and the network. For example, when the mobile station MS has been switched on and has been registered in the network, the subscriber station is in a registered and non-active mode of operation. A standby operation means SOM holds the subscriber station in such a non-active mode of operation. In such a non-active mode of operation the operation of the subscriber station MS is invoked by receiving the paging flag PF from the network control means RNC, namely when a call is pending for the subscriber station MS and when a communication connection is to be set up to the subscriber station MS.
FIG. 2 shows a general flow chart of a method for carrying-out an inter-frequency or inter-system handover in a mobile communication system when a signaling connection or a communication connection is set up. In step ST11 a handover means HORM (HandOveR Means) situated in the network control means RNC or the mobile station MS monitors the network performance regarding the capacity/coverage aspects as explained above. In step ST12 the handover means HORM decides whether in principle a handover is necessary according to the criteria determined in step ST11. If so (“Y” in step ST12), the mobile station is triggered to perform inter-frequency measurements in step ST13. More particularly, in step ST13 an IF measurement trigger signal is outputted by the handover means HORM. As indicated in FIG. 1, the IF-measurements means IFMM can be triggered by a mobile-evaluated-handover trigger signal or a network-evaluated-handover trigger signal IFTS in step ST13.
In order to perform a fast and reliable inter-frequency handover when there is the need for such a handover, it is advantageous to provide the outputting of a reliable trigger signal IFTS in either the network control means RNC and/or in the mobile station MS. Of course, in order to provide a well-designed trigger procedure, there is not a single triggering condition that needs to be monitored in step ST11 and which will eventually trigger the mobile station MS to perform IF-measurements on other frequencies or systems. Usually, a couple of conditions are monitored in step ST11 and must be fulfilled that the trigger signal is outputted in step ST13. Such conditions may for example comprise an excessively high output power from either the down-link (network to subscriber station) connection or the up-link (subscriber station to network) connection and/or a high load in the cell. If for example the network detects by measuring the uplink-interference a high load in the cell, it will attempt to trigger IF-measurements and thus a handover to a different cell or a different system. Likewise, if transmission conditions deteriorate, the mobile station MS is triggered to more and more increase its output power and therefore a high output power also indicates the need for IF-measurements and thus the need for a handover.
The prior art reference TS 25 231 V0.3.0, technical specification: Third Generation Partnership Project (3GPP); Technical specification group (TSG), radio access network (RAN); working group 1 (WG 1); Physical Layer-Measurements in the IS 95 standard, dated June 1999 (hereinafter referred to as reference [1]) describes in particular in chapters 3., 4., 5.1.2 a number of conventional measurement trigger criteria. In the mobile communication system described in reference [1] both a network handover means HORM and a subscriber station handover means HORM monitor the performance of the radio-link (RL) and can request a handover. For example, the network handover means HORM monitors the down-link by measurement reports from the subscriber station MS. The network handover means HORM also monitors the traffic load. As explained above, a hand-over evaluated by a mobile station MS is called a mobile-evaluated hand-over, abbreviated MEHO. A hand-over evaluated by the network is called a network-evaluated hand-over, abbreviated NEHO. As indicated in FIG. 1, since the mobile station MS and the network control means RNC each comprises a handover HORM each can initiate a handover according to the triggering conditions which are respectively monitored. The four basic criteria during the monitoring in step ST11 in the prior art are the “base station traffic load exceeded” condition, the “distance limits exceeded” condition, the “pilot strength below a predetermined threshold” condition and the “power level exceeded” condition as will be explained below and as is described in the aforementioned reference [1].
Firstly, regarding the condition “base station traffic load exceeded”, the network handover means HORM determines the necessity for a handover by monitoring loads at all base stations BS in the mobile communication system T1 and outputs the IF measurement trigger signal IFTS in order to balance loads between all base stations, in order to achieve a higher traffic efficiency. For example, the network handover means HORM outputs the trigger signal in step ST13 whenever the load at a base station exceeds a predetermined load threshold.
Secondly, regarding the condition “distance limits exceeded” the subscriber handover means and/or the network handover means HOM are adapted to determine the necessity for the handover on the basis of a supervision of the distance between a base station BS and the subscriber station MS. The distance between the relevant base station and the subscriber station can be determined in a synchronized system. Therefore, the trigger signal IFTS is outputted in step ST13 whenever the measured distance exceeds a predetermined distance.
Thirdly, regarding the condition “pilot strength below a predetermined threshold”, the subscriber handover means and/or the network handover means are adapted to determine the necessity for a handover on the basis of a supervision of a measured pilot signal strength falling below a predetermined power threshold. As is illustrated in FIG. 3-1 and in FIG. 4-1, in modern mobile communication systems a data transmission between a base transceiver station RBS and a subscriber station MS is carried-out by transmitting data, frames FR and the transmission frames FR consist of a control portion CP and a data portion DP. This is true for CDMA frames (FIG. 3-1) and TDMA frames in GSM (FIG. 4-1) The control portion CP consists of at least of pilot symbols and preferably also of other control symbols CS. For example, each base station BS may transmit a pilot signal of constant power on the same frequency. The subscriber station MS can monitor the received power level of the received pilot signal and can thus estimate the power loss on the connection between the base station BS and the subscriber station MS. Using the pilot signal strength for estimating the path loss, the subscriber handover means HORM outputs the trigger signal IFTS in step ST13 if the path loss is greater than a predetermined path loss threshold.
Fourthly, regarding the condition “power level exceeded” the subscriber handover means and/or the network handover means are adapted to determine the necessity for a handover on the basis of a supervision that in response to a power increase commanded by a base station BS a subscriber power adjustment module PAN (shown in FIG. 1 in the mobile station MS) is unable to further increase its power on the up-link of the communication connection CC.
FIGS. 5a–d show such a conventional adjustment of the transmission power when exchanging frames FR consisting of a number of time slots TS1 . . . TS15 between a base transceiver station (generally called node “B”) RBS and a subscriber station MS. A power adjustment module PAM in the base transceiver station (node “B”) RBS presets an upper threshold PUP, a lower threshold PDWN and an offset value POFF for the power. The power offset value POFF is used in connection with a slow power control and the upper and lower threshold values PUP, PDWN are used in connection with a fast power control in the node B.
The slower power control and the fast power control as illustrated in FIG. 5b is carried out according to the flow chart in FIG. 5c. Steps P1, P2 relate to the slow power control (the outer control loop) carried out on the RNC-side or the MS-side. In step P1 the frame error rate FER (or the block error rate BLER) is measured and in step P2 the measured FER (or the BLER) is compared with a FER target value (or a BLER target value). In step P8 a new signal interference ratio target value SIR-target is obtained. As shown in FIG. 5d, a known (simulated) relationship between a delta_SIR_target value (dB) and the logarithm of the measured FER value exists. Between two threshold values UL_delta_SIR—2 and UL_delta_SIR—1 a predetermined “working area” exists. This relationship is known, i.e. simulated beforehand. As indicated in FIG. 5d, depending on the measured value log (measured FER) a value delta_SIR_target* is read out. A new SIR_target value SIR_target is calculated according to the following equation:SIR_target=SIR_target+delta—SIR_target*Thus, the outer loop or slow power control will generate in step P8 new SIR-target values whenever steps P1, P2 are executed. The new SIR-target value is then used in the fast power control (inner loop) carried out on the node B-side or the MS-side, respectively.
In step P5 the SIR (Signal-to-Interference ratio) per slot is measured and in step P4 the measured SIR value is compared with the (current) SIR-target value as obtained in step P8. If the measured SIR value is greater than the current SIR-target value, then a decrease command is sent to the mobile station MS/network, i.e. the transmission power control parameter TPC is set to TPC=“00” in step P7. When the measured SIR value is smaller then the (current) SIR-target value in step P4, then an increase command is sent to the mobile station MS/network in step P6 by setting the transmission power control parameter TPC to TPC=“11”.
As illustrated in FIG. 5b, the slow power control and the fast power control result in a stepwise adjustment of the power Pout on the downlink DL. Since the slow power control performs steps P1, P2 for calculating the frame error rate FER (or block error rate BLER) for every frame (or block) a new SIR-target value is obtained less frequently than the fast power control carried out with steps P5, P4, P6, P7 for each slot.
The offset value Poff and the upper and lower threshold values Pup, Pdwn are also used in the power adjustment. For example, when the output power Pout exceeds the upper threshold Pup then the offset value Poff is slightly increased and when the power is lower than the lower threshold Pdwn the offset value Poff is slightly decreased. The stepwise adjustment of the power is always performed within the power range between Pdbn and Pup. Since the values Poff, Pup and Pdwn are only used for the triggering of a soft-handover, they are not of any further relevance for the present invention and any further descriptions thereof are therefore omitted.
As explained above, in the fourth condition “power level exceeded” the node B (the base station BS) commands the subscriber station MS to increase its power and if the power adjustment module PAM in the node B notices that there is no further increase of power in response to a power increase command TCP, the network handover means HORM may request a measurement by issuing the IF trigger signal.
Regarding the above described four different conditions, there are a number of significant disadvantages and some of the four described conditions can not even be implemented in future wideband code division multiple access systems (WCDMA).
Whilst reference [1] relates to the IS-95 standard and describes a synchronized CDMA system, reference [2]: TS 25.201 V2.1.0, a third generation partnership project (3GPP); technical specification group (TSG); radio access network (RAN; working group 1 (WG1); physical layer-general description, dated June 1999, describes a non-synchronized WCDMA system, in particular the multiple access used therein. In a synchronized system like the one described in reference [1] either the base station BS or the subscriber station MS can still estimate the distance between them (second trigger condition). This is possible since the chip rate on the pilot channel and all channels are synchronized (locked) to a precise system clock. This is in reference [1] accomplished by using a global positioning system (GPS). However, due to multipath propagation delay and shadowing between the base station BS and the subscriber station MS, the estimated distance may be erroneous. Therefore, the second condition “distance limits exceeded” may not be very accurate.
In condition 3 “pilot strength below a predetermined threshold” the subscriber station MS must perform measurements for triggering IF measurements and thus for triggering a handover. These continuous measurements of the pilot signal strength may drastically reduce the lifetime of the battery of the subscribers station, since the subscriber station MS must perform an average filtering of the pilot channel during a predetermined measurement time. The decrease of the lifetime of the battery is to be avoided in all circumstances, since there are already a lot of measurements that must be performed by the subscriber station, e.g. the IF measurements on other frequencies when the IF measurement trigger signal IFTS has been issued. Furthermore, the subscriber station MS has to report the pilot signal strength measurements in some form over the air-interface to the base transceiver station RBS (node B) and to the network control means RNC and this will additionally increase the interference level on the up-link UL as well as the signaling load in the network. Therefore, a load estimation according to the first condition “base station traffic load” when used in connection with the third condition “pilot strength below a predetermined threshold” may cause more signaling due to the increased signaling in an air interface of the network.
Therefore, the major disadvantage of the prior art trigger mechanisms is that some of the conditions cannot be used in synchronized or non-synchronized systems, that the lifetime of the battery is reduced and that the interference level on the up-link UL as well as the signaling load in the network is increased.
Returning to FIG. 2, in response to an IF measurement trigger signal IFTS (generated by the subscriber handover means HORM or the network handover means HORM), the subscriber station will perform IF measurements in a given time interval in step ST21. As explained above, in order to perform a fast and reliable inter-frequency handover, it is advantageous to let the subscriber station MS perform signal quality measurements on a different frequency, e.g. in a target cell or in a different system, and to report these to the network control means RNC, such that the network control means RNC can base its handover decisions, as to which cell the subscriber station MS is to be handed over, on these reported signal quality measurements. As explained below, the performing of IF-measurements in the subscriber station MS is not a trivial task. For example, in CDMA and FDMA systems the receiver of the subscriber station MS is normally busy receiving information on the current frequency and thus some measurement time has to be created in some way in such systems in order to allow inter-frequency measurements without a drastic loss of data. Conventional methods for determining a time interval in which field measurements are carried out will be described below as reference to FIGS. 3-1, 3-2, FIGS. 4-1, 4-2 and FIG. 6.
As already discussed above with reference to FIG. 3-1, in a CDMA communication system the data communication is performed by exchanging data frames FR consisting of a plurality of time slots TS1 . . . TS15. Each time slot comprises a control portion CP and a data portion DP. As described in the aforementioned reference [2] and as also indicated with step ST21′ in FIG. 3-2 and in FIG. 3-1, it also possible to carry out the data transmission in a compressed mode (also called slotted mode) in order to create some time for the IF measurement. For this purpose the network control means RNC comprises a compressed mode setting means CMSM in which the data contained in the data portion DP is compressed, i.e. concentrated to a smaller part of the frame, resulting in an idle time portion ITP. The subscriber station MS comprises a compressed mode determining means CMDM which determines i.e. realizes—being informed about the compressed mode of transmission via signaling or some information sent from the compressed mode setting means CMSM of the network control means RNC—the compressed mode of operation. If such a compressed mode of operation is detected, the subscriber station MS enters a compressed mode of operation and performs the IF measurements in the idle time IT in step 5T21″ in FIG. 3-2.
In a CDMA system such a concentration of information is achieved by reducing the processing gain G=chips/information bits=1/SF, e.g. by decreasing the spreading factor SF. Another possibility how the concentration of information can be achieved is by changing the channel coding scheme, e.g. from r=⅓ to r=½. Due to the compressed mode of operation a time interval TI is generated in which the IF measurements can be carried out by the IF measurement means IFMM in the subscribed station MS.
FIG. 4-1 and steps SC21′″ and ST21″″ show another possibility of how a time interval can be provided in which the field measurements can be carried out. In a GSM system, a specific time slot FMS of a frame consisting of a plurality of TDMA time slots TS1 . . . TS-M is specified and the field measurements are carried-out in the portion FMP. That is, in a GSM system a predetermined field measurement slot is provided in which no data is sent from the network control means or the base station transmitter to the subscriber station MS.
A further approach how an idle time interval can be provided is described in reference [1] for the case when an inter-system handover should be carried-out. In this case, as illustrated in FIG. 6, the subscriber station MS does not perform any measurements on another system and instead the other system transmits a pseudo-noise PN sequence which is received by the subscriber station MS on the same frequency on which the subscriber station MS already communicates. When the power of this PN sequence exceeds a predetermined threshold during a predetermined time, compared to other PN sequences, an inter-system handover is carried-out.
As shown in FIG. 2 and in FIGS. 3-1, 4-1, the network control means RNC triggers the mobile station and step ST13 to perform the IF measurements and it will also indicate to the subscriber station MS on which frequency belonging to a different cell or a different system said IF measurements are to be carried-out. The subscriber station SS will report the IF measurements back to the network control means RNC within a predetermined time. Then, in step ST22, the network control means RNC will determine whether a handover to the selected frequency (cell or different system) is possible. If it is not possible, because for example a too high interference is detected on the new frequency, the network control means selects a new target cell (frequency) in step ST23 and the IF measurements are repeated by the subscriber station MS in step ST21. Furthermore, the network control means RNC can order the subscriber station MS to perform a periodic search or a single search. Such a procedure is for example described in reference [1] for a synchronized communication system.
In some systems like CDMA 2000 the subscriber station MS not only reports the IF measurements back to the network control means, but it also indicates to the network control means RNC how long (time-wise) and when (the starting time) the subscriber station MS will be able for performing the desired IF measurements. If the network control means RNC has knowledge of the time-interval in which the subscriber station MS intends to perform the IF measurements, then the network control means RNC can make some provisions to compensate for data frames, which would be sent by the network control means RNC, but which the subscriber station MS would not process in the time interval in which it performs the IF measurements. That is, actually data frames will get lost in the time period in which the subscriber MS performs the field measurements unless further provisions are made.
One possibility is that the network control means RNC increases the power before or after the measurement time interval or the intervals. Since the error rate is always evaluated over a plurality of data frames, such an increase of power before and after the measurement time interval enables to keep the overall quality for error rate to an average level which will not exceed the requirements of an average error rate. On the other hand, a similar situation occurs on the side of the subscriber station MS, i.e. it will not be possible for the subscriber station MS to transmit data frames in the measurement time interval. Therefore, also the subscriber station MS may compensate possible unsent frames by increasing the power before and after the determined measurement time interval. Therefore, on the side of the subscriber station MS and on the side of the network control means RNC the quality of the received is increased. However, the above described procedures (which are generally used in CDMA 2000 and IS'95) for providing a given time interval in which the mobile station MS is to carry-out field measurements in step ST21, the PN sequence transmission and the compensation for erased frames by increasing the power, still exhibit some major drawbacks when implemented in the system as explained below.
In addition, the WCDMA procedure of carrying-out field measurements in connection with the compressed mode of operation has the following disadvantages, in particular for the system. If the spreading factor SF in the down link DL is reduced to provide the idle time interval IT in which the subscriber station MS is to perform the field measurements on other systems, the available channelization codes are reduced. That is, the hard capacity for the CDMA system is decreased.
On the other hand, if the channel coding rate is increased for a certain time period, a complicated code-rate apparatus must be implemented in the network control means RNC, since a CDMA system can carry services with different coding schemes and different interleaving depths on the same radio link.
Furthermore, the subscriber station MS has to increase its output power when measurements are performed due to the compressed mode operation, since the same data information is transmitted during a smaller time period, i.e. in the compressed data period. If the output power of the subscriber station MS and/or base transceiver station RBS would not be increased, the performance will be decreased. However, this requirement to increase the peak power of the subscriber station MS may imply a distance limitation if the subscriber station MS is already transmitting at its maximum output power. Furthermore, there is a higher risk to lose information, since the data field is not protected to the same extent when the coding rate is reduced. Therefore, on the one hand the compressed data transmission reduces the quality and on the other hand the idle time interval is quite short such that a long time is needed for carrying out the IF measurements and thus handover may be slow.
The procedure to use a PN sequence transmission as shown in FIG. 6 has the following disadvantages. In this case, all other existing mobile communication systems have to be equipped with an apparatus which transmits a PN sequence which can be detected by the subscriber station MS. This will imply high costs for the operators (and thus for the end users). Moreover, the PN sequence used in the other mobile communication systems will interfere with the CDMA systems and will reduce the capacity as well as the quality of data transmission.
The last mentioned method of increasing the power before and after the measurement time interval has the disadvantage that there is a high risk that a loss of frames due to the measurement time interval will deteriorate the speech quality in situations where speech quality is already very low, when it is likely that the subscriber station MS wants to do an inter-frequency handover close to a cell border or when the cell (sector) exhibits a high load.
A measurement time interval can be determined by the subscriber stations as the time in which no data transmission takes place from the network control means. Thus, the IF measurements cannot cause a reduction of the quality of the connection.
Summarizing the above disadvantages of providing a time interval for IF measurements according to the afore described prior art, such provisions of the measurement time interval will result in a decreased quality of service (e.g. due to loss of frames), require a complicated system modification (due to the incorporation of PN sequence generators), and will shorten the lifetime of the battery in the subscriber station MS (if the power is increased before and after the time interval). Also, the time interval is restricted by the length of the idle time in the compressed time slots.